1. Field of the Invention
The present invention generally relates to apparatuses and methods for inspecting quantum efficiency homogeneity of a solar cell. More specifically, the present invention is directed to an apparatus and a method for inspecting quantum efficiency homogeneity of a solar cell using a spatial light modulating device such as a liquid crystal display (LCD) or a digital micromirror device (DMD). The quantum efficiency homogeneity of a solar cell can be regarded equivalent to the spatial uniformity of power conversion efficiency of a solar cell.
2. Description of the Related Art
A solar cell is a device that is irradiated by sunlight to generate electric power. However, when sunlight irradiates a portion of a solar cell, only a fraction of electric power proportional to a ratio of an irradiated area to the whole area is generated in case that its quantum efficiency is uniform
In general, photocurrent generated from an irradiated portion of a solar cell may be measured when white light or laser beam is locally focused on the solar cell through a lens. Quantum efficiency homogeneity of the solar cell may be achieved by scanning the light beam and two-dimensionally mapping the photocurrent.
Since the quantum efficiency homogeneity may be deteriorated by local material defects, cracks or abnormal operations inside a solar cell, the measurement is required during research, manufacturing, and inspection of a solar cell. In addition, a lifetime of a solar cell may be predicted by observing the change of the quantum efficiency homogeneity while exposing the solar cell to a severe environment.
Conventionally, several methods have been used to inspect quantum efficiency homogeneity of a solar cell. One of the methods is that a solar cell is mounted on a two-axis translation stage and it is two-dimensionally scanned while irradiated by a light source. Inversely, a light source or a light delivering optical fiber is mounted to a two-axis translation stage and scans over the solar cell with irradiation. The light source can be either white light or monochromatic light.
Alternatively, a laser beam is launched onto a lens and an incident angle of the laser beam is mechanically or electrically controlled to change a falling position of the beam on a solar cell, and the photocurrent is measured.
However, since this method is accompanied by mechanical movements, measurement time becomes long and measurement accuracy is affected by vibration and acoustic noise.
Besides, an additional optical system must be used to control the size of an irradiating ray which determines a spatial resolution of quantum efficiency measurement. Moreover, it is not easy to control the size of the irradiating ray automatically.
Furthermore, the two-axis translation stage must be substituted with a new one having a large moving range to increase the inspection area of a solar cell.
Accordingly, there has been a need for an apparatus and a method for inspecting quantum efficiency homogeneity of a solar cell, which provides not only accuracy but also convenient control for the inspection.